The IEEE's standard for wireless LANs, designated IEEE 802.11, provides two different ways to configure a network: ad-hoc and infrastructure. In an ad-hoc network, nodes form a network “on the fly,” with each 802.11 device joining the network as it is able to send and receive signals. There is no defined structure in an ad-hoc network; there are no fixed points; and every node in the network is ideally able to communicate with every other node in the network. Although it may seem that order would be difficult to maintain in this type of network, sufficient algorithms, such as the spokesman election algorithm (SEA), are provided and are designed to “elect” one machine as the base, or master, station of the network, with the others machines being “slaves.” Another algorithm in ad-hoc network architectures uses a broadcast and flooding method to all other nodes to establish the identity of all nodes in the network.
The infrastructure architecture provides fixed network access points for communications, possibly with mobile nodes. These network access points (APs) are sometime connected to land lines to widen the LAN's capability by bridging wireless nodes to other wired nodes. If service areas overlap, handoffs may occur between wireless LANs. This structure is very similar to that used in cellular networks, however, cellular protocols are not part of the 802.11 standard.
IEEE 802.11 standard places specifications on the parameters of both the physical (PHY) and medium access control (MAC) layers of the network. The PHY layer, which actually handles the transmission of data between nodes, may use either direct sequence spread spectrum, frequency-hopping spread spectrum, or infrared (IR) pulse position modulation. IEEE 802.11 makes provisions for data rates of either 1 Mbps or 2 Mbps, and requires operation in the 2.4-2.4835 GHz frequency band, in the case of spread-spectrum transmission, which is an unlicensed band for industrial, scientific, and medical (ISM) applications; and in the 300-428,000 GHz frequency band for IR transmission. Infrared is generally considered to be more secure to eavesdropping, because IR transmissions require absolute line-of-sight links, i.e., no transmission is possible outside any simply connected space or around corners, as opposed to radio frequency transmissions, which can penetrate walls and be intercepted by third parties. However, infrared transmissions may be adversely affected by sunlight, and the spread-spectrum protocol of 802.11 does provide some rudimentary security for typical data transfers.
The MAC layer includes a set of protocols which is responsible for maintaining order in the use of a shared medium. The 802.11 standard specifies a carrier sense multiple access with collision avoidance (CSMA/CA) protocol. In this protocol, when a node receives a packet to be transmitted, it first listens to ensure no other node is transmitting. If the channel is clear, it then transmits the packet. Otherwise, it chooses a random “backoff factor,” which determines the amount of time the node must wait until it is allowed to transmit its packet. During periods in which the channel is clear, following a short waiting period, the transmitting node decrements its backoff counter. When the channel is busy it does not decrement its backoff counter. When the backoff counter reaches zero, the node transmits the packet. Because the probability that two nodes will choose the same backoff factor is small, collisions between packets are minimized. Collision detection, as is employed in Ethernet®, cannot be used for the radio frequency transmissions of IEEE 802.11, because when a node is transmitting, it cannot hear any other node in the system which may be transmitting, because its own signal will block any other signals arriving at the node. Whenever a packet is to be transmitted, the transmitting node may first send out a short ready-to-send (RTS) packet containing information on the length of the packet. If the receiving node hears the RTS, it responds with a short clear-to-send (CTS) packet. After this exchange, the transmitting node sends its packet. When the packet is received successfully, as determined by a cyclic redundancy check (CRC), the receiving node transmits an acknowledgment (ACK) packet. This back-and-forth exchange is used to avoid the “hidden node” problem, i.e., node A can communicate with node B, and node B can communicate with node C. However, node A cannot communicate node C. Thus, for instance, although node A may sense the channel to be clear, node C may in fact be transmitting to node B. The protocol described above alerts node A that node B is busy, and requires node a to wait before transmitting its packet.
Although 802.11 provides a reliable means of wireless data transfer, some improvements to it have been proposed. The use of wireless LANs is expected to increase dramatically in the future as businesses discover the enhanced productivity and the increased mobility that wireless communications can provide and as unit prices come down.
802.11 wireless LANs contain both fixed and variable parameters. The fixed parameters can not be changed for the life of the LAN instantiation. This means if conditions change e.g., traffic load, extraneous RF interference, etc., the original selection of values for the fixed parameters may become sub-optimal. Neither the 802.11 standard nor the known prior art address changing fixed 802.11 parameters during the life of a BSS.
802.11 channel RF noise can come from many sources, including overlapping BSSs and other RF radiators such as microwave ovens, lightning, etc. Depending upon the PHY in use, one channel may be noisy while another my be relatively noise-free. When a channel used by an 802.11 WLAN under the as-published 802.11 standard is noisy, many packet errors occur.
U.S. Pat. No. 5,933,420, granted Aug. 3, 1999 to Jaszewski et al., for Method and apparatus for assigning spectrum of a wireless local area network, describes use of mutually non-interfering frequencies and/or channels by overlapping BSSs depending on RF signal strength and other indicators. The reference describes use of minimally-interfering channels rather than changing frequencies and/or channels.
U.S. Pat. No. 6,049,549, granted Apr. 11, 2000 to Ganz et al., for Adaptive media control, describes a variant of the 802.11e standard wherein streams which have a history of using less of their allocated resources are polled less frequently.
U.S. Pat. No. 6,092,117, granted Jul. 18, 2000 to Gladwin et al., for System and method for automatically reconnecting a wireless interface device to a host computer, describes a technique for automatically reconnecting to a previously selected wireless host upon power up.
There are two primary solutions for 802.11 packet errors in the prior art. The first is to terminate the current BSS, reconfigure it to use another, hopefully better channel, and then create a new BSS using the newly selected channel. This procedure require manual intervention by a system administrator. The second uses an automated mechanism to monitor all channels and then change the channel used in a BSS in a coordinated fashion if channel performance degrades and a sufficiently better channel is available. Gerard Cervello, Sunghyun Choi, Stefan Mangold, and Amjad Soomro; Dynamic Channel Selection (DCS) Scheme for 802.11; Jul. 12, 2000; IEEE 802.11-00/195r2, uses a channel-switch announcement in step six of a seven-step process to change channels/frequencies across an entire BSS. This approach is different from that of the first-mentioned prior art solution in that it does not terminate the existing BSS, nor does it create a new BSS; it simply changes the channel used by a single BSS in a coordinated fashion. No fixed parameters are modified.